To ensure that emails received from a certain email address will always be detected as spam, select the messages and click the "Move to spam" button.

If you have selected the "Automatically update Black/White list when moving message to/from Spam folder." option

in "General settings",

then the sender of the selected email will be added to the Black List and if it has previously been on your White List, it will be removed from it (this applies only if such email address is not included in your contact list) and messages from this email address will from now on be considered as spam and moved to the Spam folder.

If you move a message to your Spam folder by mistake, you can remove it from the Spam folder manually. This time, the sender's email address will be removed from the Black List, and automatically added to your White List. This rule does not apply if you move it to the Trash folder.